Earlier quoted context omitted.
You replied question 2 with question 1: we don't self host email, -because- of deliverability issues.
why are they in the first place?
- when you send emails from MyWebsite.com, you want to make sure that Hotmail, Yahoo, Gmail, Proton Mail, etc users get your email.
- when you receive emails to your hi@mywebsite.com email, you don't want spam
V1 of email providers ostensibly solved this problem using lots of data as well as "supervised" machine learning (e.g. you telling the provider that a specific email is spam).
Self-hosting seems awesome if you're only handing out the email to a few people (probably a few hundred max?), you don't think it will be widely shared, and it's a slightly unique email that makes it hard for a script to guess (e.g. "hello@mywebsite.com" will be easily guessed but "john2@mywebsite.com" may not be)